ANOMALOUS ACTIVITIES OF TROPICAL CYCLONE OVER THE WESTERN NORTH PACIFIC AND THE RELATED LARGE-SCALE CIRCULATION FEATURES DURING 1998 AND 1999

Based on the data from the Joint Typhoon Warning Center (JTWC), the activities of tropical cyclone (TC) in the western North Pacific (WNP) have been studied. The results showed that the TC anomalous activities in these two years were very similar in such a way that the typhoon number was far less than normal and the location was westward off the mean. The analyses of the main factors resulting in these anomalies showed that during the period from April 1998 to 1999, the patterns of the monsoon trough, vertical wind shear and sea-level pressure were all not favorable to the genesis of typhoon in the eastern WNP, which was an important reason causing the decrease of T Y number and westward movement of genesis location. And in which the anom alous westward and northward movements of monsoon trough in midsummer exerted a main effect on the TC anoma lous activities. The further analyses of the tropical large-scale anomalous circulation have shown that the anomalies of the circulation over the WNP were the main reason leading to the anomalies of the monsoon trough and vertical wind shear. And the 1999 anomalous circulation may be caused by the response of the tropical atmosphere to the anomalous heat sources around Indonesia. Because of the anomalous heating, the eastward propagating Kelvin waves were exited in its eastern side (eastern WNP) and the corresponding anomalous wind field displayed the pattern that the anomalous easterlies prevailed in the high troposphere and the anomalous westerlies in the low level. This led to the almost reverse change of vertical wind shear in the eastern and western WNP. On the other hand, the anomalous strong easterlies in the low troposphere were not favorable to the eastward extension of the monsoon westerlies, which led to the anomalous westward movement of the monsoon trough so that it can not reach the region east of Philippine where the TC frequently generate clim atologically.